1. Field of the Invention
The present invention relates to a laser diode, and more particularly, to a laser diode generating passive mode locking that does not contain non-linear sector of a saturable absorber (SA), and a method of creating an optical pulse using the same diode.
2. Description of the Related Art
An optical pulse is utilized in a variety of fields such as a clock reproduction by a 3R reproducer (re-amplifying, re-shaping, re-timing) in optical communication, a light source in the optical communication, a light source for light sampling, a carrier for radio frequency (RF) communication. Particularly, when an optical pulse is produced using a semiconductor laser diode, it is very advantageous in aspects of the size and economics of a device. Therefore, lots of researches on a pulse laser diode have been made.
Physical mechanism that causes a laser diode to emit a pulse is Q-switching, gain switching, mode beating, or mode locking. Since an RF signal is used in case of the Q-switching or gain switching, there are limitations due to the speed of an electronic device and a response speed of a laser diode. In case of the mode beating, a laser diode having a plurality of distributed feedback (DFB) sectors is generally used. However, a very precise process is required to form these DFB sectors.
The mode locking is roughly divided into active mode locking and passive mode locking. The active mode locking modulates to the same frequency as a mode interval using a modulator within the laser diode and performs phase locking (meaning fixing a phase difference between modes). Since the active mode locking performs the phase locking using an external RF signal, it has the same problems as those of the Q-switching.
Therefore, the passive mode locking is preferred. In case of the passive mode locking, a non-linear sector of a saturable absorber (SA) is input in the laser diode to perform the phase locking. However, since the passive mode locking includes the non-linear sector of the SA, it has a complicated structure. Also, according to a prior art passive mode locking, the width of a variable frequency is small, so that there is manufacturing difficulty. For example, when a reference frequency is 40 GHz, the width of a variable frequency is less than 1 GHz (2.5%). Therefore, it is difficult to adjust a frequency to a reference frequency by applying a current after forming a device. Therefore, a device should be precisely manufactured. This problem also occurs in relation to the active mode locking.
Also, there is a laser diode including a single DFB sector and using the mode beating. The laser diode using such mode beating includes a DFB sector, a phase control sector, and a gain sector, and creates an optical pulse using mode beating occurring between compound cavity modes. In detail, when a current equal to or greater than a threshold current is applied to the DFB sector, the DFB sector operates as a single mode laser to emit light a phase control sector and a gain sector. The emitted light passes through the phase control sector and the gain sector, and is reflected by an as-cleaved facet (a cut plane of the end of the gain sector) to return back to the DFB sector. By such reflection, the entire device does not operate as a single mode but oscillates in two modes. The device creates an optical pulse using beating between the two modes.
The laser diode using the mode beating does not use an SA but uses only two modes and therefore has limitation in creating various optical pulses.
A research on a passive mode locking laser diode using a distributed brag reflector (DBR) is under progress. As reported, a normal wave guide structure does not create a pulse swiftly. However, when the wave guide is formed in an almost square, a pulse is swiftly created. In more detail, the cross-section of a wave guide of a general laser diode has a size of 1 μm in horizontal length and 0.3 μm in vertical length. On the other hand, a wave guide for the passive mode locking has an almost square-shaped cross-section of 0.7 μm in horizontal length and 0.5 μm in vertical length.
Currently, interaction between non-linear modes such as four wave mixing (FWM) is drawing attention as a reason generating the passive mode locking. The wave guide structure having the square cross-section also increases only a light restriction coefficient of the wave guide to increase the non-linear effect. Here, the light restriction coefficient describes a degree a wave guide restricts light.
However, it is particularly difficult to manufacture a wave guide having a shape close to a square in case of the laser diode using the DBR.